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Colorado Department of Transportation

2017 Pavement Design Manual

196

PRINCIPLES OF DESIGN FOR FLEXIBLE PAVEMENT

6.1 Introduction

Design of flexible pavement structures involves the consideration of numerous factors, the most

important are truck volume, weight and distribution of axle loads, HMA, underlying material

properties, and the supporting capacity of the subgrade soils. Typical reconstruction projects

should have a design life of 20 years for reconstructions and 10 years for rehabilitations unless mitigating circumstances exist.

Methods are presented in this section for the design of the flexible pavement structure with respect

to thickness of the subbase, base, surface courses, and the quality and strength of the materials in

place. Interaction between pavement materials and climate is evaluated as part of the M-E Design

process.

6.2 M-E Design Methodology for Flexible Pavement

M-E Design uses an iterative process. The key steps in the design process include the following:

1. Select a Trial Design Strategy

2. Select Appropriate Performance Indicator Criteria for the Project: Establish

criteria for acceptable pavement performance (i.e. distress/IRI) at the end of the design

period. Performance criteria were established to reflect different magnitudes of key

pavement distresses which trigger major rehabilitation or reconstruction. CDOT

criteria for acceptable performance is based on highway functional class and location.

3. Select Appropriate Reliability Level for the Project: The reliability is in essence a

factor of safety that accounts for inherent variations in construction, materials, traffic,

climate, and other design inputs. The level of reliability selected should be based on

the criticality of the design and selected for each individual performance indicator.

CDOT criteria for a desired reliability is based on highway functional class and

location.

4. Assemble All Inputs for the Pavement Trial Design Under Consideration: Define

subgrade support, asphalt concrete and other paving material properties, traffic loads,

climate, pavement type and design, and construction features. The inputs required to

run the M-E Design program may be obtained using one of three hierarchical levels

and need not be consistent for all inputs in a given design. The hierarchical level for a

given input is selected based on the importance of the project, input, and resources at

the disposal of the user.

5. Run the M-E Design Software: The software calculates changes in layer properties,

damage, key distresses, and IRI over the design life. The key steps include:



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a) Processing input to obtain monthly values of traffic, seasonal variations of

material, and climatic inputs needed in design evaluations for the entire design

period.

b) Computing structural responses (stresses and strains) using multilayer elastic

theory or finite element based pavement response models for each axle type

and load and each damage-calculation increment throughout the design

period.

c) Calculating accumulated distress at the end of each analysis period for the

entire design period.

d) Predicting key distresses (rutting, bottom-up/top-down fatigue cracking, and

thermal cracking) at the end of each analysis period throughout the design life

using calibrated mechanistic-empirical performance models.

e) Predicting IRI as a function of initial IRI, distresses accumulating over time,

and site factors at the end of each analysis increment.

6. Evaluate Adequacy of the Trial Design: The trial design is considered “adequate”

if none of the predicted distresses/IRI exceed the performance indicator criteria at the

design reliability level chosen for the project. If any criteria has been exceeded, one

must determine how the deficiency can be remedied by altering material types,

properties, layer thicknesses, or other design features.

7. Revise the Trial Design, as Needed: If the trial design is deemed “inadequate”, one

must revise the inputs and re-run the program until all performance criteria have been

met. Once met, the trial design becomes a feasible design alternative.

Design alternatives that satisfy all performance criteria are considered feasible from a structural

and functional viewpoint and may be considered for other evaluations, such as life cycle cost

analysis. Consultation of the mix design(s) with the RME shall occur. A detailed description of

the design process is presented in the interim edition of the AASHTO Mechanistic-Empirical

Pavement Design Guide Manual of Practice, AASHTO 2008.

6.3 Select a Trial Design Strategy

6.3.1 Flexible Pavement Design Types

Figure 6.1 Asphalt Concrete Pavement Layer Systems illustrates well known CDOT

combinations of asphalt concrete structural pavement layers. Designers can select from among

several flexible pavement options as shown below:

Conventional Flexible Pavements: Flexible pavements consisting of a relatively thin

asphalt concrete layer placed over an unbound aggregate base layer and subgrade.

Deep-Strength AC Pavements: Flexible pavements consisting of a relatively thick

asphalt concrete layer placed over an unbound aggregate base layer and subgrade.



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Full-Depth AC Pavements: Asphalt concrete layers placed directly over the subgrade.

Figure 6.1 Asphalt Concrete Pavement Layer Systems

The asphalt concrete layer in Figure 6.1 Asphalt Concrete Pavement Layer Systems may be

comprised of several layers of asphalt concrete courses to include a surface course, intermediate

or binder course, and a base course (see Figure 6.2 Structural Layers). The surface, binder, and

base courses are typically different in composition and are placed in separate construction

operations (3).

Surface Course: The surface course normally contains the highest quality materials.

It provides characteristics such as friction, smoothness, noise control, rut and shoving

resistance, and drainage. It also serves to prevent the entrance of excessive quantities

of surface water into the underlying HMA courses, bases, and subgrade.

Intermediate/Binder Course: The intermediate course, sometimes called binder

course, consists of one or more lifts of structural HMA placed below the surface course.

Its purpose is to distribute traffic loads so stresses transmitted to the pavement

foundation will not result in permanent deformation to the course. It also facilitates the

construction of the surface course.

Base Course: The base course consists of one or more HMA lifts located at the bottom

of the structural HMA course. Its major function is to provide the principal support of

the pavement structure. The base course should contain durable aggregates that will

not be damaged by moisture or frost action.



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Figure 6.2 Structural Layers

6.3.2 Concept of Perpetual Pavements

A perpetual pavement is defined as an asphalt pavement designed and built to last longer than 50

years without requiring major structural rehabilitation or reconstruction, and needing only periodic

surface renewal in response to distresses confined to the top of the pavement (6). Full depth and

deep-strength asphalt pavement structures have been constructed since the 1960s. Full-depth

pavements are constructed directly on subgrade soils and deep-strength sections are placed on

relatively thin (4 to 6 inches) granular base courses. A 20-year traffic design period is to be used

for the traffic loading. One of the chief advantages of these pavements is that the overall section

of the pavement is thinner than those employing thick granular base courses. Such pavements

have the added advantage of significantly reducing the potential for fatigue cracking by

minimizing the tensile strains at the bottom of the asphalt layer (7) (see Figure 6.1 Asphalt

Concrete Pavement Layer Systems). An asphalt perpetual pavement structure is designed with

a durable, rut and wear resistant top layer with a rut resistant intermediate layer and a fatigue

resistant base layer (see Figure 6.3 Perpetual Pavement Design Concept)

Rigid Foundation

Natural Subgrade

Compacted Subgrade

Unbound Subbase

Unbound Base

Asphalt Base

Asphalt Binder

Asphalt Surface

Asphalt Concrete

Layer

Unbound Layer

Subgrade

Road bed



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Figure 6.3 Perpetual Pavement Design Concept

This concept may be used in conventional, deep strength, or full depth asphalt structural layering.

In mechanistic design, the principles of physics are used to determine a pavement’s reaction to

loading. Knowing the critical points in the pavement structure, one can design against certain

types of failure or distress by choosing the right materials and layer thicknesses (7). Therefore,

the uppermost structural layer resists rutting, weathering, thermal cracking, and wear. SMAs or

dense-graded SuperPave mixtures provide these qualities. The intermediate layer provides rutting

resistance through stone-on-stone contact and durability is imparted by the proper selection of

materials. Resistance to bottom-up fatigue cracking is provided by the lowest asphalt layer having

a higher binder content or by the total thickness of pavement reducing the tensile strains in this

layer to an insignificant level (6).

6.3.3 Establish Trial Design Structure

The designer must establish a trial design structure (combination of material types and

thicknesses). This is done by first selecting the pavement type of interest (see Figure 6.5 M-E

Design Software Screenshot Showing General Information (left), Performance Criteria and

Reliability (right)). M-E Design automatically provides the top layers of the selected pavement

type. The designer may add or remove pavement structural layers and/or modify the layer material

type and thickness as appropriate. Figure 6.4 M-E Design Software Screenshot of Flexible

Pavement Trial Design Structure shows an example of flexible pavement trial design with

pavement layer configuration on the left and layer properties of the AC surface course on the right.

Asphalt Surface

Intermediate Layer

Asphalt Base

Subgrade or

Unbound Base

Zone of High

Compression

(4 to

6 inches)

Max Tensile

Strain

Temperature

Dep

th

Change in Temperature

High Quality HMA or SMA

High Performance PG Binder

High Modulus, Rut Resistant

High-Temp PG Binder (same as surface)

Low-Temp PG Binder (+1 temp grade

higher than surface)

Flexible, Fatigue Resistant

High-Temp PG Binder (appropriate

or depth)

Low-Temp PG Binder (+1 temp grade

higher than surface)

Recommended

Thickness

Mixture Type

and Binder Grade

1 ½ to 3 inches

4 to 7 inches

3 to 4 inches



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Figure 6.4 M-E Design Software Screenshot of Flexible Pavement Trial Design Structure

6.4 Select the Appropriate Performance Indicator Criteria for the Project

Table 2.4 Recommended Threshold Values of Performance Criteria for New Construction

or Reconstruction Projects presents recommended performance criteria for flexible pavement

design. The designer should enter the appropriate performance criteria based on functional class.

An appropriate initial smoothness (IRI) is also required, For new flexible pavements, the

recommended initial IRI is 50 inches/mile.

Figure 6.5 M-E Design Software Screenshot Showing General Information (left)

Performance Criteria and Reliability (right) shows performance criteria for a sample flexible

pavement trial design. The coefficients of performance prediction models considered in the design

of a new flexible pavement are shown in Figure 6.6 Performance Prediction Model Coefficients

for Flexible Pavement Designs (Marshall Mix) through Figure 6.8 Performance Prediction

Model Coefficients for Flexible Pavement Designs (PMA Mix). The value of AC rutting

coefficient (BR1) is based on the type of HMA



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Figure 6.5 M-E Design Software Screenshot Showing General Information (left),

Performance Criteria and Reliability (right)



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Figure 6.6 Performance Prediction Model Coefficients for Flexible Pavement Designs

(Marshall Mix)



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(Superpave Mix)



205


(PMA Mix)



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6.5 Select the Appropriate Reliability Level for the Project

Recommended reliability levels for flexible pavement designs are located in Table 2.3 Reliability

(Risk). The designer should select an appropriate reliability level based on highway functional

class and location. Figure 6.5 M-E Design Software Screenshot Showing General Information

(left), Performance Criteria and Reliability (right) shows design reliability values for a sample

flexible pavement trial design.

6.6 Assemble M-E Design Software Inputs

6.6.1 General Information

6.6.1.1 Design Period

The design period for new flexible pavement construction and reconstruction is at least 20 years.

For special designs, the designer may use a different design period as appropriate.

6.6.1.2 Construction Dates and Timeline

The following inputs are required to specify the construction dates and timeline (see Figure 6.5

M-E Design Software Screenshot Showing General Information (left), Performance Criteria

and Reliability (right)):

Base/subbase construction month and year

Pavement construction month and year

Traffic open month and year

The designer may select the most likely month and year for construction completion of the key

activities listed above. Selection is based on the designer’s experience, agency practices, or

estimated from the planned construction schedule. For large projects that extend into different

paving seasons, it is suggested each paving season be evaluated separately and the designer judge

the acceptability of the trial design based on the more conservative situation. The M-E Design

software does not consider staged construction events, nor does it consider the impact of

construction traffic on damage computations.

Note: The pavement performance predictions begin from the month the pavement is open to

traffic. The changes to pavement material properties due to time and environmental conditions are

calculated beginning from the month and year the material was placed.

6.6.1.3 Identifiers

Identifiers are helpful in documenting the project location and recordkeeping. M-E Design allows

designers to enter site or project identification information such as the location of the project (route

signage, jurisdiction, etc.), identification numbers, beginning and ending milepost, direction of

traffic, and date.



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6.6.2 Traffic

Several inputs are required for characterizing traffic for the M-E Design software and are described

in detail in Section 3.1 Traffic.

6.6.3 Climate

The climate input requirements for the M-E Design software are described in detail in Section 3.2

Climate.

6.6.4 Pavement Layer Characterization

As shown in Figure 6.2 Structural Layers, a typical flexible pavement design comprises of the

following pavement layers: asphalt concrete, unbound aggregate base layers, and subgrade. The

inputs required by M-E Design for characterizing these layers are described in the following

sections.

6.6.4.1 Asphalt Concrete Characterization

Asphalt concrete types used in Colorado include:

Hot Mix Asphalt (HMA): Composed of aggregates with an asphalt binder and certain

anti-stripping additives.

Stone Matrix Asphalt (SMA): Gap-graded HMA that maximizes rutting resistance

and durability with a stable stone-on-stone skeleton held together by a rich mixture of

AC, filler, and stabilizing agents.

The designers should apply the following guidelines when defining an asphalt concrete layer:

As much as possible and as appropriate, the asphalt concrete layers must be combined

into three layers: surface, intermediate and base. Asphalt layers with similar HMA

mixtures may be combined into a single layer.

When multiple layers are combined, the properties of the combined layer should be the

weighted average of the individual layers.

The M-E Design software does not consider very thin layers (thickness less than 1.5

inches).

Weakly stabilized asphalt materials (i.e. sand-asphalt) should not be considered an

asphalt concrete layer.

M-E Design models layer by layer rutting. Table 6.1 Layered Rut Distribution

shows the percentages used for calculating the final rutting in Colorado.



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Table 6.1 Layered Rut Distribution

Layer Colorado

Percent Distribution

Global

Percent Distribution

Hot Mix Asphalt 60 80

Aggregate Base Course 10 5

Subgrade 30 15

Designers are required to input volumetric properties such as air voids, effective asphalt content

by volume, aggregate gradation, mix density, and asphalt binder grade (see Figure 6.9 Asphalt

Concrete Layer and Material Properties in M-E Design). The designers are also required to

input the engineering properties such as the dynamic modulus, creep compliance, indirect tensile

strength of HMA materials, and the viscosity versus temperature properties of rolling thin film

oven (RTFO) aged asphalt binders. These inputs can be obtained following the input hierarchy

levels depending on the criticality of the project. The volumetric properties entered into the

program need to be representative of the in-place asphalt concrete mixture. The project-specific

in-place mix properties will not be available at the design stage. The designer should use typical

values available from previous construction records or target values from the project

specifications.

Figure 6.9 Asphalt Concrete Layer and Material Properties in M-E Design



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Table 6.2 Input Properties and Recommendations for HMA Material Characterization presents the HMA input requirements of the M-E Design Method and recommendations for

obtaining inputs at each hierarchical input level. The designer may use Level 1 inputs of typical

CDOT HMA mixtures for Level 2 and 3 inputs. See APPENDIX F and Table 2.6 Selection of

Input Hierarchical Level for selection of an appropriate hierarchical level for HMA

characterization. For new construction (i.e. new HMA) the designer should always click “True”

for the Possion’s Ratio (currently the default value is “False”).

Table 6.2 Input Properties and Recommendations for HMA Material Characterization

Input Property Level 1 Level 2 Level 3

Dynamic Modulus (E*)

Mix specific E*

and/or AASHTO

TP62 test results

Gradation (APPENDIX E)

Asphalt Binder Properties Binder properties from laboratory testing

of HMA using AASHTO T315

Binder grade

(APPENDIX E)

Tensile Strength 1 at 14 oF AASHTO T322

test results

Use tensile strength and creep compliance

(APPENDIX E) Creep Compliance

Poisson’s Ratio M-E Design software option

(Is Poisson's ratio calculated?) Use 0.35

Air Voids Use air voids (APPENDIX E)

Volumetric Asphalt

Content Use volumetric asphalt content (APPENDIX E)

Total Unit Weight Use total unit weight (APPENDIX E)

Surface Shortwave

Absorptivity Use 0.85

Coefficient of Thermal

Contraction of the Mix

1.3E-05 in./in./°F (mix CTE)

and 5.0 E-06 in./in./°F (aggregate CTE)

Thermal Conductivity 0.67 Btu/(ft)(hr)(oF)

Heat Capacity 0.23 BTU/lb.- ˚F

Reference Temperature 70 ˚F

Note: 1 The designer should use Level 1 Inputs. The Level 3 Inputs for tensile strength are much smaller which will

cause more thermal cracking and greater creep compliance.

6.6.4.2 Unbound Layers and Subgrade Characterization

Refer to Section 5.3.1 Unbound Layer Characterization in M-E Design for unbound aggregate

base layer characterization. Refer to Section 4.4 Subgrade Characterization for M-E Design

for subgrade characterization.



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6.7 Run M-E Design Software

Designers should examine all inputs for accuracy and reasonableness prior to running the M-E

Design software. Next, one should run the software to obtain outputs required to determine if the

trial design is adequate. After a trial run has been successfully completed, M-E Design will

generate a report in form of a PDF and/or Microsoft Excel file, refer to Figure 6.10 Sample

Flexible Pavement Trial Design PDF Output Report. The output report has input information,

reliability of design, material properties, and predicted performance. It also includes the month to

month estimates of material properties over the entire design period in either tabular or graphical

form. For a flexible pavement trial design, the report provides the following:

Monthly estimates of HMA dynamic modulus for each sublayer

Monthly estimates of resilient modulus of unbound layers and subgrade

Monthly estimates of AADTT

Monthly estimates of climate parameters

Cumulative trucks (FHWA Class 4 through 13) over the design period

Cumulative ESALs over the design period (an intermediate file in the project folder)

After the trial run is complete, the designer should re-examine all inputs and outputs for accuracy

and reasonableness before accepting a trial design as complete.

Figure 6.10 Sample Flexible Pavement Trial Design PDF Output Report



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6.8 Evaluate the Adequacy of the Trial Design

The output report of a flexible pavement trial design includes the monthly accumulation of the

following key distress types at their mean values and chosen reliability for the entire design period:

Alligator Fatigue Cracking: Traditional wheel path cracking that initiates at the

bottom of the HMA layer and propagates to the surface under repeated load

applications. Beyond a critical threshold, the rate of cracking accelerates and may

require significant repairs and lane closures. Fatigue cracking is highly dependent on

the effective asphalt content by volume and air voids.

Transverse Cracking: Thermal cracks typically appear as transverse cracks on the

pavement surface due to low temperatures, hardening of the asphalt, and/or daily

temperature cycles. Excessive transverse cracking may adversely affect ride quality.

The designer should examine the results to evaluate if the performance criteria for each of the

above-mentioned indicators are met at the desired reliability. If alligator fatigue cracking or

transverse cracking criteria have not been met, the trial design is deemed unacceptable and

revised accordingly to produce a satisfactory design.

The output report also includes the monthly accumulation of the following secondary distress types

and smoothness indicators at their mean values and chosen reliability for the entire design period:

Permanent Deformation: The report includes HMA rutting and total permanent

deformation (includes rutting on unbound layers and subgrade). Excessive rutting may

cause safety concerns.

Surface-Initiated Fatigue Cracking or Longitudinal Cracking: These load-related

cracks appear at the HMA surface and propagate downwards. Beyond a critical

threshold, the rate of cracking accelerates and may require significant repairs and lane

closures.

IRI: The roughness index represents the profile of the pavement in the wheel paths.

Higher IRI indicates unacceptable ride quality.

The designer should examine the results to evaluate if the performance criteria for

permanent deformation, surface-initiated fatigue cracking or longitudinal cracking, and IRI

meet the minimum of 12 years at the desired reliability. If any of the criteria have not been

met, the trial design is deemed unacceptable and revised accordingly to produce a satisfactory

design.

Another important output is the reliability level of each performance indicator at the end of the

design period. If the reliability value predicted for the given performance indicator is greater than

the target/desired value, the trial design passes for that indicator. If the reverse is true, then the

trial design fails to provide the desired confidence and the performance indicator will not reach the



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critical value during the pavement’s design life. In such an event, the designer needs to alter the

trial design to correct the problem.

The strategies for modifying a trial design are discussed in Section 6.9 Modifying Trial Designs.

The designer can use a range of thicknesses to optimize the thickness of the trial design to make it

more acceptable. In addition, the software allows the designer to perform a sensitivity analysis on

the key inputs. The results of the sensitivity analysis can be used to further optimize the trial

design if modifying AC thickness alone does not produce a feasible design alternative. A detail

description of thickness optimization procedure and sensitivity analysis is provided in the Software

HELP Manual.

6.9 Modifying Trial Designs

An unsuccessful trial design may require revisions to ensure all performance criteria are satisfied.

The trial design is modified by systematically revising the design inputs. In addition to layer

thickness, many other design factors influence performance predictions. The design acceptance is

distress-specific; in other words, the designer needs to first identify the performance indicator that

failed to meet the performance target and modify one or more design inputs that has a significant

impact on the given performance indicator. The impact of design inputs on performance indicators

is typically obtained by performing a sensitivity analysis. Strategies used to produce a satisfactory

design by modifying design inputs can be broadly categorized into to following:

Pavement layer considerations

Increasing layer thickness

Modifying layer type and layer arrangement

Foundation improvements (i.e. stabilize the upper subgrade soils)

Pavement material improvements:

Use of higher quality materials (i.e. use of polymer modified asphalt, crushed

stones)

Material design modifications (i.e. increase asphalt content, reduce amount of

fines, modify gradations etc.)

Construction quality (i.e. reduce HMA air voids, increase compaction density,

decrease as-constructed pavement smoothness)

Once again, when modifying the design inputs, the designer needs to be aware of the sensitivity

of these inputs to various distress types. Changing a single input to reduce one distress may result

in an increase in another distress. For example, the designer may consider using a harder asphalt

to reduce HMA rutting, but that will likely increase the predicted transverse cracking. Table 6.3

Modifying Flexible Pavement Trial Designs presents a summary of inputs that may be modified

to optimize trial designs and produce a feasible design alternative.



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Table 6.3 Modifying Flexible Pavement Trial Designs

Distress/IRI Design Inputs that Impact

AC Rutting

Use a polymer modified asphalt for the HMA surface layer

Increase the dynamic modulus of the HMA mixture(s)

Reduce the asphalt content in the HMA mixture(s)

Increase the amount of crushed aggregate

Increase the amount of manufactured fines in the HMA mixture

Transverse

Cracking

Decrease the stiffness of the AC surface mix

Use a softer asphalt

Increase asphalt binder

Increase indirect tensile strength

Reduce creep compliance

Increase AC layer thickness

Alligator

Cracking

Increase HMA layer thickness

Increase HMA dynamic modulus for HMA layers thicker than 5 inches

and decrease HMA dynamic modulus for HMA layers thinner than 3

inches

Revise the mixture design of the HMA base layer

Increase asphalt binder content

Achieve higher density and lower air voids during compaction

Use harder asphalt/polymer modified asphalt but ensure good

compaction is achieved

Increase percent manufactured fines, and/or percent crushed

aggregates

Reduce stiffness gradients between upper and lower layers

Using a higher quality/stiffer HMA layer on top of poor

quality/low resilient modulus granular base or foundation tends to

increase fatigue cracking

Increase the thickness or stiffness of a high quality unbound base layer

and/or use a stabilized layer

Unbound Base

Rutting

Increase the resilient modulus of the aggregate base

Increase the density of the aggregate base

Stabilize the upper foundation layer for weak, frost susceptible, or

swelling soils

Place a layer of select embankment material with adequate compaction

Increase the HMA or granular layer thickness

Address drainage related issues to protect from the detrimental effects

of moisture

Subgrade

Rutting

Increase the layer stiffness and layer thickness of any layers above the

subgrade layers:

Increase HMA and/or unbound layer thickness or stiffness

Include a stabilized drainable base



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Distress/IRI Design Inputs that Impact

Improve the engineering properties of the subgrade material:

Increase the stiffness (modulus) of the subgrade layer(s) itself

through the use of lime stabilized subgrade

Effective use of subsurface drainage systems, geotextile fabrics,

and impenetrable moisture barrier wraps to protect from the

detrimental effects of moisture

Increase the grade elevation to increase the distance between the

subgrade surface and ground water table

IRI

Reduce initial IRI (achieving smoother as-constructed pavement

surface through more stringent smoothness criteria)

Improve roadbed foundation (replace frost susceptible or expansive

subgrade with non-frost susceptible or stabilized subgrade materials)

Place subsurface drainage system to remove ground water

Figure 6.11 Sensitivity of HMA Alligator Cracking to Truck Volume through Figure 6.32

Sensitivity of HMA IRI to Base Thickness. Figure 6.30 Sensitivity of HMA IRA to AC

Thickness presents sensitivity plots of a sample flexible pavement trial design showing the effects

of key inputs, such as traffic volume, asphalt binder content, asphalt binder grade, air voids, base

type, base thickness, and climate on key distresses/IRI. Note: The plots do not exhaustively cover

the effects of all key factors on flexible pavement performance; other significant factors are not

shown herein.

Figure 6.11 Sensitivity of HMA Alligator Cracking to Truck Volume



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Figure 6.12 Sensitivity of HMA Alligator Cracking to AC Thickness

Figure 6.13 Sensitivity of HMA Alligator Cracking to Asphalt Binder Content



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Figure 6.14 Sensitivity of HMA Alligator Cracking to Air Voids

Figure 6.15 Sensitivity to HMA Alligator Cracking to Base Type



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Figure 6.16 Sensitivity of HMA Alligator Cracking to Base Thickness

Figure 6.17 Sensitivity of HMA Alligator Cracking to Climate



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Figure 6.18 Sensitivity of Total Rutting to Truck Volume

Figure 6.19 Sensitivity of Total Rutting to AC Thickness



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Figure 6.20 Sensitivity of Total Rutting to Asphalt Binder Grade

Figure 6.21 Sensitivity of Total Rutting to Air Voids



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Figure 6.22 Sensitivity of Total Rutting to Base Type

Figure 6.23 Sensitivity of Total Rutting to Climate



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Figure 6.24 Sensitivity of HMA Transverse Cracking to Thickness

Figure 6.25 Sensitivity of HMA Transverse Cracking to Asphalt Binder Grade



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Figure 6.26 Sensitivity of HMA Transverse Cracking to Asphalt Binder Content

Figure 6.27 Sensitivity of HMA Transverse Cracking to Base Type



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Figure 6.28 Sensitivity of HMA Transverse Cracking to Climate

Figure 6.29 Sensitivity of HMA IRI to Truck Volume



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Figure 6.30 Sensitivity of HMA IRI to AC Thickness

Figure 6.31 Sensitivity of HMA IRI to Asphalt Binder Grade



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Figure 6.32 Sensitivity of HMA IRI to Base Thickness

6.10 HMA Thickness with ABC

As a minimum, the designer should include 4 inches of ABC for any thickness of HMA when

the design truck traffic is less than 1,000 trucks per day. Six inches of ABC should be used

for any thickness of HMA when the design truck traffic is greater than 1,000 trucks per day.

6.11 Required Minimum Thickness of Pavement Layer

Compaction of a hot mix asphalt pavement during its construction is the single most important

factor affecting the ultimate performance of the pavement. Achieving adequate compaction

increases pavement performance by decreasing rutting, reducing damage due to moisture and

oxidation, and increasing the stability of the mix. Factors affecting the cooling rate of the mat

include the layer thickness, the temperature of the mix when placed, ambient temperature,

temperature of the base, and wind conditions. Layer thickness is the single most important variable

in the cooling rate of an asphalt mat, especially for thin layers. This is especially true in cool

weather because thin layers of an asphalt mat have less capacity to retain heat than thicker lifts of

pavement. The thicker layers of an asphalt mat help to maintain the temperature at a workable

level, thus increasing the time available for compaction. Because of the increased difficulty in

achieving density and the importance of achieving compaction, a minimum layer thickness for

construction of HMA pavement is two inches. A designer of special mixes, such as stone matrix



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asphalt or thin lift HMA should look at minimum thickness requirements of the particular product.

The minimum thickness of these special mixes is likely to be a dimension other than two inches.

6.12 Asphalt Materials Selection

6.12.1 Aggregate Gradation

Definitions of Aggregate Size:

Nominal Maximum Aggregate Size (NMAS): The size of aggregate of the smallest

sieve opening through which the entire amount of aggregate is permitted to pass.

Note: For Item 403 - HMA and SMA, the Nominal Maximum Size is defined

as one sieve size larger than the first sieve to retain more than ten percent of the

aggregate.

Maximum Aggregate Size is defined as one size larger than nominal maximum size.

The flexible pavement usually consists of ¾ inch nominal maximum aggregate size

(NMAS) in the lower layers, with a hot mix asphalt (HMA) Grading S. The top surface

layer, should be either stone matrix asphalt (SMA) or a Grading SX. SMA mixes are

often used in areas expected to experience extreme traffic loading. When low to high

traffic loads are expected, a ½ inch NMAS, Grading SX should be used.

CDOT uses the No. 30 sieve as one of the job-mix formula tolerance sieves. Table 6.4 Master

Range Table for Stone Mix Asphalt. is based (with some exceptions) on NCHRP No. 4 and 3/8

inch and AASHTO ½ inch and ¾ inch SMA gradations ranges, where the No. 30 sieve range is

included in the ½ inch and ¾ inch gradations.

SMA Gradation Nomenclature Example:

The ¾ inch (19.0 mm) gradation is named the ¾ inch Nominal Maximum Aggregate Size gradation

because the first sieve that retains more than 10 percent is the ½ inch sieve, and the next sieve

larger is the ¾ inch sieve, refer to Table 6.4 Master Range Table for Stone Mix Asphalt.

A CDOT study (1) found less thermal segregation in the top lift when Grading SX mixes were

used. HMA Grading SX can also be used where layers are very thin or where the pavement must

taper into an existing pavement. A study from Auburn University (2) found little difference in the

stability or rutting of ¾ inch and ½ inch NMAS mixes. CDOT cost data for 2005 showed a slight

increase in the cost per ton of Grading SX mixes as compared to Grading S mixes with the same

bid quantities.

HMA with a 1-inch NMAS, Grading SG, should not be used in the surface layer. Although

Grading SG mixes have been used in specialized situations, they are not currently used or accepted

on a regular basis for pavement mixes. CDOT has found that the production and placement of

Grading SG mixes are prone to segregation and the use should be discouraged.



227

Table 6.4 Master Range Table for Stone Matrix Asphalt

Sieve Size

Percent by Weight Passing Square Mesh Sieves

#4

(4.75 mm)

Nominal

Maximum

3/8”

(9.5 mm)

Nominal

Maximum

1/2”

(12.5 mm)

Nominal

Maximum

3/4”

(19.0 mm)

Nominal

Maximum

1 “ (25 mm) - - - 100

¾” (19.0 mm) - - 100 90-100

½” (12.5 mm) 100 100 90-100 50-88 3/8” (9.5 mm) 100 90-100 50-80 25-60

#4 (4.75 mm) 90-100 26-60 20-35 20-28

#8 (2.36 mm) 28-65 20-28 16-24 16-24

#16 (1.18mm) 22-36 - - -

#30 (600 µm) 18-28 12-18 12-18 12-18

#50 (300 µm) 15-22 10-15 - -

#100 (150 µm) - - - -

#200 (75 µm) 12-15 8-12 8-11 8-11

For structural overlays, the minimum allowed layer thickness will be 2 inches. For functional

overlays used in preventive maintenance or other treatments, thinner lifts are allowed.

Table 6.5 HMA Grading Size and Location Application and Table 6.6 HMA Grading Size

and Layer Thickness gives guidance for mix selection and recommended layer thicknesses for

various layers and nominal maximum aggregate sizes.

Table 6.5 HMA Grading Size and Location Application

CDOT

HMA Grade

Nominal

Maximum

Aggregate

Size (NMAS)

Application

SF No. 4 sieve Leveling course, rut filling, scratch course, etc.

ST 3/8 inch Thin lifts and patching

SX ½ inch Top layer (preferred)

S ¾ inch Top layer, layers below the surface, patching

SG 1 inch Layers below the surface, deep patching



228

Table 6.6 HMA Grading Size and Layer Thickness

CDOT

HMA Grade

Nominal

Maximum

Aggregate Size

(NMAS)

Structural/Overlay

Layer Thickness (inches)

Minimum Maximum

SX ½ inch 2.00 3.00

S ¾ inch 2.25 3.50

SG 1 inch 3.00 4.00

Functional Overlay

Layer Thickness (Inches)

SF No. 4 sieve 0.751 1.50

ST 3/8 inch 1.125 2.50

Note: 1 Layers of SF mixes may go below 1 inch as needed to taper thin lift

to site conditioning (i.e. rut filling).

6.12.2 Selection of SuperPaveTM Gyratory Design

To choose the appropriate number of revolutions of a SuperPave™ gyratory asphalt mix design

on a particular project, determining the design 18k ESALs and the high temperature environment

for the project is necessary. The following steps should be followed to determine the proper

SuperPave™ gyratory design revolutions for a given project:

Step 1. Determine 18k ESALs: In order to obtain the correct SuperPave™ gyratory compaction

effort (revolutions), the 18k ESALs must be a 20-year cumulative 18k ESAL of the design

lane in one direction. The compaction effort simulates the construction compaction roller

to obtain the correct voids properties to resist the intended traffic in the design lane. The

department’s traffic analysis unit of the Division of Transportation Development (DTD)

automatically provides an ESAL calculator. One must use a 20-year design, appropriate

number of lanes, and a specified flexible pavement. Even a 10-year asphalt overlay must

use a 20-year cumulative 18k ESAL number for the design lane.

Step 2. Reliability for the 7-Day Average Maximum Air Temperature: The next decision is

to determine the type of project being designed. For new construction or reconstruction,

asphalt cement with 98 percent reliability for low and high temperature properties is

recommended. For overlays, asphalt cement with 98 percent reliability for high

temperature properties (rutting resistance) and 50 percent reliability for low temperature

properties (cracking resistance) is recommended. Asphalt cements with lower than 98

percent reliability against rut resistance should not be specified. In the SuperPave™

system, anything between 50 percent and 98 percent reliability is considered 50 percent

reliability for the purpose of binder selection. The low temperatures are specified at a

lower reliability for overlays because of reflection cracking.



229

Step 3. Determine Weather Data for the Project: Obtain the highest 7-day average maximum

air temperature, based on weather data in the project area from the computer program

LTPPBind 3.1 (beta). Refer to Section 6.12.3 Binder Selection for a further explanation

of LTPPBind 3.1 (beta). From the appropriate high temperature, find the environmental

category for the project from Table 6.7 Environmental Categories. The Environmental

Categories are from CDOT Pavement Management Program’s Environmental Zones.

The Environmental Zones (Categories) are one of four pavement groupings used to group

pavements into families that have similar characteristics.

Table 6.7 Environmental Categories

Highest 7-Day Average Air

Temperature

High Temperature

Category

> 97°F

(> 36°C)

Hot

(southeast and west)

> 88° to 97°F

(> 31° to 36°C)

Moderate

(Denver, plains and west)

81° to 88°F

(27° to 31°C)

Cool

(mountains)

< 81°F

(< 27°C)

Very Cool

(high mountains)

Step 4. Selection of the Number of Design Gyrations (NDES): Select the NDES from Table 6.8

Recommended SuperPaveTM Gyratory Design Revolution (NDES). For example,

Table 6.7 shows that for 5,000,000 18k ESALs and a high temperature category of

“Cool”, the design revolutions should be 75.

Table 6.8 Recommended SuperPaveTM Gyratory Design Revolution (NDES)

CDOT Pavement

Management System

Traffic Classification

(20 Year Design ESAL)

20 Year Total

18k ESAL in the Design

Lane

High Temperature Category

Very Cool Cool Moderate Hot

Low < 100,000 50 50 50 50

100,000 to < 300,000 50 75 75 75

Medium 300,000 to < 1,000,000 75 75 75 75

1,000,000 to < 3,000,000 75 75 75 100

High 3,000,000 to < 10,000,000 75 75 100 100

Very High 10,000,000 to < 30,000,000 --- --- 100 ---

Very Very High ≥ 30,000,000 --- --- 125 ---

Note: Based on Standard Practice for SuperPave™ Volumetric Design for Hot-Mix Asphalt (HMA), AASHTO

Designation R 35-04.



230

6.12.3 Binder Selection

Performance graded (PG) binders have two numbers in their designation, such as PG 58-34. Both

numbers describe the pavement temperatures in degrees Celsius at which the pavement must

perform. The first number (58 in the example) is the high temperature standard grade for the

pavement, and the second number (minus 34 in the example) is the low temperature standard grade.

PG 64-28 (rubberized) or PG 76-28 (polymerized) or bituminous mixtures should only be placed

directly on an existing pavement or milled surface that does not show signs of stripping or severe

raveling. Cores should be taken to determine if stripping is present. Because of a limited number

of tanks, Colorado local suppliers only have the capacity to supply a limited number of asphalt

cement grades. Table 6.9 Available Asphalt Cement Grades in Colorado shows available

grades that maybe used and/or available on CDOT projects.

Table 6.9 Available Asphalt Cement Grades in Colorado

Polymer Modified Unmodified

PG 76-28

PG 70-28

PG 64-28

PG 58-34

PG 64-22

PG 58-28

Note: The Region Materials Engineer may select a different gyratory design

revolution for the lower HMA lifts.

LTPPBind 3.1 (beta) is a working version, dated September 15, 2005. Beta only means it is going

through the 508-compliance process for the visually disabled users as required by the Federal

Government. The computer program may be obtained from the following web address:

http://www.fhwa.dot.gov/pavement/ltpp/ltppbind.cfm

The program allows the user to select the asphalt binder grade for the appropriate project site

conditions. In the Preferences under the File menu, use 12.5mm ( ½ inch) for the CDOT target rut

depth default value. The computer program has a help menu to assist the user and supporting

technical information regarding the computation of design temperatures required for the selection

of the asphalt binder grade as provided in the Climatic Data and Algorithms sections. The

algorithms are broken down under four subsections. Each algorithm equation is shown and briefly

explained for high temperature, low temperature, PG with depth, and PG grade bumping.

High Temperature: The high temperature is based on a rutting damage model. The

LTPP high temperature model was not used in this version since it provided very

similar results to the SHRP Model at 98 percent reliability. Initially, the user must

select a preference for a target rut depth, but they have the option to change the target

rut depth. The default is 12.5 mm (½ inches).

Low Temperature: The low temperature is based on LTPP climatic data using air

temperature, latitude, and depth to surface.

http://www.fhwa.dot.gov/pavement/ltpp/ltppbind.cfm



231

PG with Depth: LTPP pavement temperature algorithms were used to adjust the PG

for a depth into the pavement. The LTPP algorithms are empirical models developed

from seasonal monitoring data.

PG Grade Bumping: PG grade bumping was based on the rutting damage concept

for high temperature adjustments. Adjustments were developed as the difference

between PG for standard traffic conditions (ESAL of 3 million and high speed) and

site conditions. 187 sites throughout the U.S. for five target rut depths were analyzed.

The PG adjustments were then averaged by various ESAL ranges, traffic speeds, and

Base PG.

The following steps should be followed to determine the proper SuperPave™ asphalt cement grade

for a given project:

Step 1. Determine Proper Reliability to Satisfy Pavement Temperature Property

Requirements: The first step is to determine what type of project is being designed.

For new construction or reconstruction, asphalt cement with 98 percent reliability

for both low and high pavement temperature properties is recommended.

For overlays, asphalt cement with 98 percent reliability for high pavement

temperature properties (rutting resistance) and 50 percent reliability for low

pavement temperature properties (cracking resistance) are recommended.

Asphalt cements with lower than 98 percent reliability against rut resistance should

not be specified.

In the SuperPave™ system, anything between 50 and 98 percent reliability is

considered 50 percent reliability for the purpose of binder selection.

The low pavement temperatures are specified at a lower reliability for overlays

because of reflection cracking.

Refer to Figure 6.33 PG Binder Grades for a graphical representation of

reliability.



232

Figure 6.33 PG Binder Grades

Step 2. Determine Weather Data for the Project: Obtain the SuperPave™ recommended

asphalt cement grade, based on weather data and traffic in the project area.

Recommendations on 98 percent reliability high and low pavement temperature weather

stations are found in Figure 6.34 Colorado 98 Percent Reliability LTPP High

Pavement Temperature Weather Station Models and Figure 6.35 Colorado 98

Percent Reliability LTPP Low Pavement Temperature Weather Station Models,

neither of which accounts for grade bumping. The program also calculates the reliability

of various asphalt cements for a given location. This source will yield the 98 and 50

percent reliability asphalt cement for a project area with a free flowing traffic condition,

which is described in Step 3. For example, when the recommendations call for a PG

58-22 for a given project, due to the available binder grades in Colorado, a PG 64-22

would be specified. This selection provides for rut resistance while preserving the same

level of resistance to cracking. Because of the danger of rutting, in no case should the

recommended high temperature requirements be lowered based on availability. Each

RME has a copy of this program.

Step 3. Select Location of Roadway: Place the cross hair on the location of area of interest in

the weather data program LTPP Bind. The program selects five weather stations

surrounding the area of interest. The designer has the option to use any number of

weather stations representative of the climate at the area of interest.

Step 4. Adjust HMA Performance Grade Binder to Meet Layer Depth, Traffic Flow and

Loading Requirements: SuperPave™ high temperature reliability factors are based on

historical weather data and algorithms to predict pavement temperature. At a depth layer

of 1 inch or more below the surface, high temperature recommendations are changed

because of the depth and temperatures at that depth.



233

For pavements with multiple layers a lesser grade may be specified for lower layers based

on the amount of material needed and other economical design decisions. In many cases,

the requirements for lower layers might be obtained with an unmodified or more

economical grade of asphalt cement. It is recommended that at least 10,000 tons of mix

in the lower layer is needed before a separate asphalt cement is specified for the lower

layer.

Adjustments can be made to the base high temperature binder through the ‘PG Binder

Selection’ screen. Adjustments to reliability, depth of layer, traffic loading, and traffic

speed (fast and slow) will be required. These adjustments are called grade bumping.

Additional grade bumping may be performed for stop and go traffic characteristics such

as intersections. This extra grade bump may be applied, but is suggested the designer

have prior regional experience on doing such.

6.12.3.1 Example

Example: A new roadway project will be constructed near Sugarloaf Reservoir. It will have two

lanes per direction and a traffic characteristic of slow moving because it is a winding mountain

road. Find the appropriate binder grade. NDES for the surface layer is obtained in the same manner

as the previous example and has a design revolution of 75.

Step 1. Determine 18k ESAL: Design Lane ESALs = 4,504,504 from DTD web site (20 year

18k ESAL in the design lane).

Step 2. Use LTPP Software Database: Use LTPPBind software database to obtain the data from

the nearest weather station, Sugarloaf Reservoir. Appropriate weather stations can be

determined from information on state, county, coordinates, location, and/or station ID.

Figure 6.36 LTPP Interface Form for Weather Station Selection (Version 3.1) is

where the cross hair is placed for the new roadway project. Figure 6.37 LTPP Weather

Station Output Data (Version 3.1) shows the data at the weather station Sugarloaf

Reservoir.

Step 3. Select the Desired Weather Stations: The LTPPBind software gives the option to select

the weather stations that provide the best weather data at the project location (see the upper

table in Figure 6.38 LTPP PG Binder Selection at 98 Percent Reliability). Check the

first three weather stations. Uncheck the two weather stations furthest from the project,

these stations are too far from the site and not representative of site conditions.

Step 4. Select the Temperature Adjustments: Because this is a principal arterial and a new

construction project, 98 percent reliability is chosen with a layer depth of zero (0) for the

surface layer (see Figure 6.38 LTPP PG Binder Selection at 98 Percent Reliability).

Step 5. Select the Traffic Adjustments for High Temperature: Select the appropriate traffic

loading and traffic speed. The design lane ESALs are 4,504,504 and the traffic speed is

slow. Grade bumping is automatic and is demonstrated by toggling in appropriate cells.



234

The following data summarized in Table 6.10 SuperPaveTM Weather Data Summary

are obtained from Steps 1 through 5.

Step 6. Select Final Binder: Table 6.8 Available Asphalt Cement Grades in Colorado lists

the binder grades available in Colorado. A PG 58-28 (unmodified) is available, but it does

not meet the low temperature requirement. The lowest temperature binders available in

Colorado can meet is -34 C. This is available in PG 58-34 (polymer modified).

Therefore, at 98 percent reliability use PG 58-34.

Step 7. Find the Temperature that Falls into the Environmental Category: Use Table 6.10

Environmental Categories (restated) to obtain the highest 7-day average air

temperature, 24.3°C. Table 6.11 Environmental Categories (restated) shows the

temperature falls into the category ‘Very Cool’ (high mountains).

Step 8 Select the Gyratory Design Revolution (NDES): Table 6.11 Recommended

SuperPaveTM Gyratory Design Revolution (NDES) shows at 4,504,504 18k ESAL and

a high temperature category of “Very Cool” the design revolutions should be 75.



235

Figure 6.34 Colorado 98 Percent Reliability LTPP High Pavement Temperature Weather Station Models



236

Figure 6.35 Colorado 98 Percent Reliability LTPP Low Pavement Temperature Weather Station Models



237

Figure 6.36 LTPP Interface Form for Weather Station Selection (Version 3.1)

Figure 6.37 LTPP Weather Station Output Data (Version 3.1)



238

Table 6.10 SuperPaveTM Weather Data Summary

98 Percent Reliability

Depth of Layer 0 mm

Traffic Loading and Speed Adjustment 10.3°C (slow)

PG Binder Grade 52 -34

Table 6.11 Environmental Categories (restated)

Highest 7-Day Average Air

Temperature

High Temperature

Category

> 97°F

(> 36°C)

Hot

(southeast and west)

> 88° to 97°F

(> 31° to 36°C)

Moderate

(Denver, plains and west)

81° to 88°F

(27° to 31°C)

Cool

(mountains)

< 81°F

(< 27°C)

Very Cool

(high mountains)

Table 6.12 Recommended SuperPaveTM Gyratory Design Revolution (NDES) (restated)

CDOT Pavement

Management System

Traffic Classification

(20 Year Design ESAL)

20 Year Total

18k ESAL in the Design

Lane

High Temperature Category

Very

Cool Cool Moderate Hot

Low < 100,000 50 50 50 50

100,000 to < 300,000 50 75 75 75

Medium 300,000 to < 1,000,000 75 75 75 75

1,000,000 to < 3,000,000 75 75 75 100

High 3,000,000 to < 10,000,000 75 75 100 100

Very High 10,000,000 to < 30,000,000 --- --- 100 ---

Very Very High ≥ 30,000,000 --- --- 125 ---

Note: Based on Standard Practice for SuperPave™ Volumetric Design for Hot-Mix Asphalt (HMA), AASHTO

Designation R 35-04.



239

6.12.4 Asphalt Binder Characterization for M-E Design

For flexible pavement design using M-E Design, the viscosity of the asphalt binder is a critical

input parameter to incorporate the viscoelastic response (i.e. time-temperature dependency) of

asphalt concrete mixtures. The asphalt binder viscosity is used in the calculations of dynamic

modulus values of asphalt mixtures for aged and unaged conditions. The key input parameters

that define the viscosity temperature relationship are the slope (A) and intercept (VTS) resulting

from a regression of the asphalt binder viscosity values measured or estimated at different

temperatures.

Laboratory testing of asphalt binders is required to develop viscosity temperature relationships at

the Level 1 input hierarchy. For performance grade binders, the asphalt binder viscosity values

can be estimated from the dynamic shear rheometer test data conducted in accordance with

AASHTO T 315, Determining the Rheological of Asphalt Binder Using a Dynamic Shear

Rheometer (DSR). Alternatively, for conventional grade binders (i.e. penetration grade or

viscosity grade), the asphalt binder viscosity values can be obtained from a series of conventional

tests, including absolute and kinematic viscosities, specific gravity, softening point, and

penetrations. At the hierarchical input Level 3, the default values of A-VTS parameters included

in M-E Design are based on the asphalt binder grade selection.

For flexible pavement rehabilitation designs, the age-hardened binder properties can be determined

using asphalt binder extracted from field cores of asphalt pavement layers that will remain in place

after rehabilitation. For projects where asphalt is not extracted, historical information and data

may be used. Table 6.13 Recommended Sources of Inputs for Asphalt Binder

Characterization presents recommended sources for asphalt binder characterization at different

hierarchical input levels. Refer to the AASHTO Intrim MEPDG Manual of Practice and MEPDG

Documentation for more information.

Table 6.13 Recommended Sources of Inputs for Asphalt Binder Characterization

Materials

Category Measured Property

Recommended

Test Protocol

Hierarchical Input

Level

3 2 1

Asphalt

Binder

Asphalt binder complex shear modulus (G*)

and phase angle (δ); at 3 test temperatures, or

AASHTO T 315

Conventional binder test data: Penetration, or AASHTO T 49

Ring and ball softening point

Absolute viscosity

Kinematic viscosity

Specific gravity, or

AASHTO T 53

AASHTO T 202

AASHTO T 201

AASHTO T 228

Brookfield viscosity AASHTO T 316

Asphalt binder grade: PG grade, or AASHTO M 320

Viscosity grade, or AASHTO M 226

Penetration grade AASHTO M 20

Rolling thin film oven aging AASHTO T 315



240

6.13 Asphalt Mix Design Criteria

6.13.1 Fractured Face Criteria

CDOT’s aggregate fractured face criteria requires the aggregate retained on the No. 4 sieve must

have at least two mechanically induced fractured faces (2) (see Table 6.14 Fracture Face

Criteria).

Table 6.14 Fractured Face Criteria

Percent Fractured Faces

of 20 Year 18k ESAL

in Design Lane

SF ST SX S SG SMA

Non-Interstate Highways

or

Pavements with

< 10,000,000 Total 18K ESALs

60% 60% 60% 60% 90% 90%

Interstate Highways

or

Pavements with

> 10,000,000 Total 18K ESALs

70% 70% 70% 70% 90% 90%

6.13.2 Air Void Criteria

A design air void range of 3.5 to 4.5 percent with a target of 4.0 percent will be used on all SX, S,

SG, and ST mixes. A design air void range of 4.0 to 5.0 percent with a target of 4.5 percent will

be used on all SF Mixes. Refer to Table 6.15 Minimum VMA Requirements for design air voids

and minimum VMA requirements and criteria for voids at NDES. The air void criteria will be

applied to the approved design mix. The nominal maximum size is defined as one size larger than

the first sieve to retain more that 10 percent. The designer should interpolate specified VMA

values for design air voids between those listed in the table. All mix designs shall be run with a

gyratory compactor angle of 1.25 degrees. CDOT Form #43 will establish construction targets for

asphalt cement and all mix properties at air voids up to 1.0 percent below the mix design optimum.

The designer should extrapolate VMA values for production (CDOT Form 43) air voids beyond

those listed in Table 6.15 Minimum VMA Requirements.



241

Table 6.15 Minimum VMA Requirements

Nominal Maximum Size1

mm (in)

Design Air Voids 2, 3

3.5% 4.0% 4.5% 5.0%

37.5 (11/2”) 11.6 11.7 11.8

N/A 25.0 (1”) 12.6 12.7 12.8

19.0 (3/4”) 13.6 13.7 13.8

12.5 (1/2”) 14.6 14.7 14.8

9.5 (3/8”) 15.6 15.7 15.8 16.9

Note: 1 The nominal maximum size defined as one size larger than the first sieve to retain more than 10%. 2 Interpolate specified VMA values for design air voids between those listed. 3 Extrapolate specified VMA values for production air voids between those listed.

6.13.3 Criteria for Stability

Criteria for stability and voids filled with asphalt (VFA) are shown in Table 6.16 Criteria for

Stability and Voids Filled with Asphalt (VFA).

Table 6.16 Criteria for Stability and Voids Filled with Asphalt (VFA)

SuperPave™ Gyratory

Revolutions (NDES)

Hveem Minimum

Stability* VFA (%)

125 30 65-75

100 30 65-75

75 28 65-80

50 ** 70-80

Note: 1-inch mix (CDOT Grade SG) has no stability requirements.

* Hveem Stability criteria for mix design approval and for field verification.

** Hveem Stability is not a criterion for mixes with a NDES of 50.

6.13.4 Moisture Damage Criteria

Moisture damage criteria are shown in Table 6.17 Moisture Damage Criteria.

Table 6.17 Moisture Damage Criteria

Characteristic Value

Minimum dry split tensile strength, (psi) 30

Minimum tensile strength ratio, CP-L 5109, (%) 80

Minimum tensile strength ratio, CP-L 5109, SMA, (%) 70



242

6.14 Effective Binder Content (By Volume)

Effective binder content (Pbe) is the amount of binder not absorbed by the aggregate, i.e. the amount

of binder that effectively forms a bonding film on the aggregate surfaces. Effective binder content

is what the service performance is based on and is calculated based on the aggregate bulk specific

gravity (Gsb) and the aggregate effective specific gravity (Gse). The higher the aggregate

absorption, the greater the difference between Gse and Gsb. The effective binder content by volume

is the effective binder content (Pbe) times the ratio of the bulk specific gravity of the mix (Gmm)

and the specific gravity of the binder (Gb). The formula is:

Pbe (by volume) = Pbe * (Gmm /Gb)

Where

Pbe = effective asphalt content, percent by total weight of mixture

Gmm = bulk specific gravity of the mix

Gb = specific gravity of asphalt (usually 1.010)

Pbe is determined as follows:

Pbe = Pb – (Pba/100) * Ps

Where

Pb = asphalt, percent by total weight of mixture

Pba = absorbed asphalt, percent by total weight of aggregate

Ps = aggregate, percent by total weight of mixture

Pba is determined as follows:

Pba = 100 ((Gse – Gsb)/ (Gsb *Gse)) *Gb

Where

Pba = absorbed asphalt, percent by total weight of aggregate

Gse = effective specific gravity of aggregate

Gsb = bulk specific gravity of aggregate

6.15 Rumble Strips

When Rumble Strips are installed, they shall be of the style and location as shown on CDOT’s

Standard Plans, M & S Standards, July 2012 Plan Sheet No. M-614-1, Rumble Strips.



243

References

1. Gilbert, K., Thermal Segregation, 2005, CDOT Report No. CDOT-DTD-R-2005-16.

http://www.coloradodot.info/programs/research/pdfs/2005/thermalsegregation.pdf..

2. Brown, E.R. and Bassett, C.E. Effects of Maximum Aggregate Size on Rutting Potential

and Other Properties of Asphalt-Aggregate Mixtures 1990. Transportation Research

Record 1259, TRB, National Research Council, Washington, D.C., 1990, pp. 107-119.

3. HMA Pavement Mix Type Selection Guide, FHWA Information Series 128, Federal

Highway Administration, 400 7th Street S.W., Washington, DC 20590 and National

Asphalt Pavement Association (NAPA), 5100 Forbes Blvd, Lanham, MD 20706, January

2001.

4. Guide for Mechanistic-Empirical Design, Final Report, NCHRP Project 1-37A, National

Cooperative Highway Research Program, Transportation Research Board, National

Research Council, Submitted by ARA, INC., ERES Consultants Division, Champaign, IL,

March 2004.

5. Locander, Robert, Analysis of Using Reclaimed Asphalt Pavement (RAP) as a Base

Material, Final Report, CDOT Report No. CDOT-2009-5, Colorado Department of

Transportation, February 2009.

6. Perpetual Pavements - A Synthesis. Asphalt Pavement Alliance, 5100 Forbes Boulevard,

Lanham, MD 20706 : Asphalt Pavement Alliance, 1/2002. Synthesis.

7. Perpetual Bituminous Pavements. Transportation Research Board (TRB). 2101

Constitution Avenue, NW, Washington, DC 20418 : Transportation Research Board,

National Research Center, December 2001. Research Circular. TRB Committee on

General Issues in Asphalt Technology (A2D05). Number 503, ISSN 0097-8515.

8. AASHTO Mechanistic-Empirical Pavement Design Guide, A Manual of Practice, Interim

Edition, July 2008, American Association of State Highway and Transportation Officials,

Washington, DC, 2008.

http://www.coloradodot.info/programs/research/pdfs/2005/thermalsegregation.pdf

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